1. Field of the Invention
The present invention relates to an image determining apparatus and an image determining method that determine whether image data read from a document by a reading device is a color image or a monochrome image.
2. Description of the Related Art
A color copier has a function of ACS (Auto Color Select). This is a function to determine if a document to be copied is in color or monochrome. However, the ACS function might fail in determination. Particularly when a monochrome document is erroneously determined as a color document, the cause of the erroneous determination may include a phenomenon called color shift. Though a monochrome document was read, a color component caused by the color shift is mixed in the image data of a reading result, and thereby the determination fails. The reason why the phenomenon of color shift occurs will be described below.
An imaging sensor mounted on a color copier typically comprises three line sensors for R, G and B that divide reflected light from a color document into R (red), G (green) and B (blue) to read each color component.
For the purpose of the above-described spectroscopy, a spectroscopic filter is typically installed on each line sensor. These line sensors have physical clearances between each color. Thus, the image read at the same time is read at different positions on the document by a distance corresponding to the above-described clearance. Therefore, in order to form final color image data from image data read by each line sensor, it is only necessary to make position correction for the above-described clearance.
However, if an uneven scanning speed in reading the document or an accidental shock during the scanning due to an external cause occurs, the RGB lines, which should intrinsically form an image, cannot appropriately form an image, and the phenomenon of color shift occurs.
If the color shift occurs at an image edge portion of a monochrome document, though R and G are low brightness signals exhibiting black, for example, B forms a line with high brightness exhibiting white. If they are formed into an image, blue data (fake color) is generated on the data intrinsically having only monochrome information.
The fact that a color component is mixed in the monochrome document due to the phenomenon called color shift was described. If the color component is included, the ACS function may erroneously determine a monochrome document as a color document. Japanese Patent Laid-Open No. 2009-260640 solves this determination problem using three technologies.
The first technology determines the pixel as monochrome even if a target pixel has some color components. This technology provides a first threshold value, and if the maximum color difference of the target pixel is smaller than the first threshold value, the pixel is determined as monochrome.
The second technology determines the read-out document as a monochrome document if the number of color pixels to the entire read-out image data is small. This technology provides a second threshold value, and if the number of color pixels is smaller than the second threshold value, the read-out document is determined as a monochrome document.
The third technology lowers sensitivity of color determination for a part of a region in a document. This technology provides a third threshold value, and if the number of color pixels in the region is smaller than the third threshold value, the region is determined as monochrome. This technology determines all the regions in the document, respectively, and finally determines the document as a color document or a monochrome document.
Here, the part of the region refers to a region where the color shift becomes particularly large. In an apparatus that feeds a document using an automatic document feeder or the like and reads the document, for example, the document collides against various members on a feeding path. The various members include a feeding roller, a guide plate and the like. If the document collides against such members, an image forming relationship of the respective RGB line sensors is largely shifted due to the above-described accidental shock, and whereby the color component is generated.
The Japanese Patent Laid-Open No. 2009-260640, for the region where the color shift due to an accidental shock as above occurs, determines a monochrome document as monochrome using the above-described third technology.
However, a color image on a document, which is desired to be actually determined as a color document by the ACS, is an image in which color pixels are continuously lumped to some degrees such as a character, for example. The Japanese Patent Laid-Open No. 2009-260640 sets the threshold value larger for the region with large color shift and determines a monochrome document with the color shift as monochrome on the basis of whether the number of counted color pixels exceeds the pixel number or not. This method cannot discriminate the color component generated by the color shift from the color component of a color image. Thus, even if a color image which should be intrinsically detected is present in a region with large color shift, if the number of color pixels is small, it is likely that the document is not determined as a color document.